The present invention is concerned with an improved electric motor system employing motors of the wound rotor type, and is more particularly concerned with a variable speed motor system which is adapted to enable a polyphase motor to start without excessive current, which is further adapted to increase the available power output and/or torque of the motor, and which is operative to regulate the motor speed so as to maintain the speed at a selected fixed value regardless of the load on the motor.
As is well known, polyphase motors of the induction type characteristically include a stator winding which is energized by a polyphase power source operative to produce a rotating magnetic field adjacent the motor rotor, which field is in turn operative to induce voltages in the rotor to generate current in the rotor windings which causes the rotor to follow the rotating field. The current flow generated in the rotor when the rotor is still at rest is very large and, as the rotor gains speed, the difference between its speed and that of the rotating field decreases so that the current in the rotor correspondingly decreases in magnitude. The main reason for the high current at start is that the stator magnetic field cannot penetrate the rotor due to the opposing magnetic field generated by the high current induced in the low resistance winding on the rotor. Moreover the starting current lags the voltage by a large phase angle. For this reason, when the rotor is of the wound type, it is customary to connect the rotor windings to slip rings which are in turn connected to a starter comprising a plurality of variable resistors which are connected in series with the rotor windings respectively to provide appropriate starting conditions and, when the motor has attained its normal working speed, the resistors forming the starter are disconnected and the slip rings are short-circuited so that the rotor will then function essentially as a squirrel-cage rotor during normal operation.
Since the current in the rotor winding of a conventional induction motor is generated by transformer action from the stator, the motor stator is burdened not only by the need to generate its own magnetic field and the need to supply power overcoming its own losses, but it must also generate sufficient current in the rotor to create the magnetic field of the rotor and to supply the rotor losses as well. Under starting conditions, moreover, the motor stator must supply the power which is dissipated in the variable resistors which constitute the conventional starter arrangement. These characteristics of conventional polyphase induction motors of the wound rotor type have required that comparatively high levels of current flow be effected in the stator coils, but have nevertheless provided a practical limit on the power level which can be supplied to the motor rotor and, in addition, have limited the available power output and torque of the motor.
The present invention is intended to overcome these and other problems of the prior art.